Elimination of paternal mitochondrial DNA in intraspecific crosses during early mouse embryogenesis.

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Abstract
To examine whether mtDNA is uni- or biparentally transmitted in mice, we developed an assay that can detect sperm mtDNA in a single mouse embryo. In intraspecific hybrids of Mus musculus, paternal mtDNA was detected only through the early pronucleus stage, and its disappearance co-incided with loss of membrane potential in sperm-derived mitochondria. By contrast, in interspecific hybrids between M. musculus and Mus spretus, paternal mtDNA was detected throughout development from pronucleus stage to neonates. We propose that oocyte cytoplasm has a species-specific mechanism that recognizes and eliminates sperm mitochondria and mtDNA. This mechanism must recognize nuclearly encoded proteins in the sperm midpiece, and not the mtDNA or the proteins it encodes, because sperm mitochondria from the congenic strain B6.mtspr, which carries M. spretus mtDNA on background of M. musculus (B6) nuclear genes, were eliminated early by B6 oocytes as in intraspecific crosses. We conclude that cytoplasmic genomes are transmitted uniparentally in intraspecific crosses in mammals as in Chlamydomonas and that leakage of parental mtDNA is limited to interspecific crosses, which rarely occur in nature.